Within semiconductor systems, it is generally necessary to match the impedance of a transmission line (e.g., a transmission channel) with the corresponding termination impedance of a termination resistor in order to prevent undesirable signal reflections. Such signal reflections act as noise on the transmission line in relation to signals subsequently transmitted on the transmission line. Termination resistors of conventional semiconductor modules or systems are often disposed outside semiconductor chips constituting the semiconductor modules or systems. However, in the event that the termination resistors are disposed outside high performance semiconductor chips such as double data rate 2 (DDR2) synchronous dynamic random access memory (SDRAM) chips, there may be some limitations in preventing undesirable signal reflections.
Recently, termination resistors have been provided inside high performance semiconductor chips to prevent undesirable signal reflections. That is, on-die termination (ODT) circuits have been widely used in semiconductor modules and/or semiconductor systems. The ODT circuits include switching circuits, which are turned on or off to control currents that flow therein. Thus, power consumption of the semiconductor modules including ODT circuits may be reduced as compared with semiconductor modules including termination resistors disposed outside the semiconductor chips. Resistance values of the ODT circuits may vary according to process/voltage/temperature (PVT) conditions. Hence, it may be necessary to calibrate the resistance values of the ODT circuits using impedance calibration circuits before the ODT circuits are utilized.
The impedance calibration circuits may employ a comparator that compares a reference voltage with a resistance value of a resistor (e.g., an external resistor) connected to a pad so as to generate a pull-up code signal and a pull-down code signal for calibrating a resistance value of the ODT circuit. The external resistor connected to the pad may have a constant resistance, e.g., 240Ω, regardless of the PVT condition.